Canon 15-85 IS USM VS. Sigma 17-50 f2.8 DG OS HSM

Hello all,

I need all your thoughts about my decision about getting the new Sigma 17-50 f2.8 OS DG HSM even though I already have the Canon 15-85 f3.5-5.6 IS USM. I already pre-ordered the Sigma 17-50 but i have the choice to cancel it as soon as possible.

The main reason of buying the Sigma is mainly because the Canon 15-85 IS has some limitations on low-light conditions due to its aperture 3.5-5.6. I never had a lens with a fixed f2.8 aperture so i guess that's the reason why some of my low-light shots are mostly dark. I am using a Canon 7D. I've read all the reviews of the Sigma 17-50 OS DG HSM and looks like it has a decent performance comparable to Canon 17-55 IS USM but with a huge price difference.

Can you all help me justify my decision or just ust a waste of money?

Please help.

All your thoughts will be greatly appreciated

Best,

Bensho